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Pratical Hardware Questions for DIY Electronic Load

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t1d:
I am building my own version of Jay’s Dynamic Electronic Load. https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/  I have several practical hardware questions…

For the DUT Input and DMM Output wires, from the banana sockets, on the case, to the PCB, what type of wire should I use? Twisted pair? Coax?

I use a switch, to select between the Reference Voltage and Auxiliary Function Generator inputs. As this switch will pass the signal, itself, what type of wire does it require? And, should I use some type of switching circuit, on the PCB, instead of taking the signal off of the PCB, running it through wires and the switch? If a switching mechanism on the PCB is best, what type of switching circuits might I consider?

I know that I need to separate the power circuit (ground plane) from the signal board(s) (ground plane) and I plan to do that. But, should the voltage reference be on its own, separate board (ground plane), too?

This is a rough draft, of my circuit. I have not proofed it, yet, but you can get the idea.

Thank you, for your help.


Kleinstein:
The switch for choosing between the internal reference and the external generator signal is not critical, as the signal level is relatively high there and the signal is low impedance. So those simple lever switches are OK.
One could consider a circuit that does not need the switch at all, if the internal and external signal are just added. So the internal reference would be still used to adjust the offset and leaving the input open is enough to disable an external signal.

There should be no special need to use separate ground planes. It is more like on should avoid a ground plane at all, especially with the high current paths at the output. This type of circuit is more suitable to use a star ground point. A ground plane is more for higher frequency and less suitable for precision.

A point too look for might be protection of the ground side of the F-gen input and the current monitoring output. These are connected to the power side and with wrong use could possibly carry high currents. So it might be a good idea to have something like a fuse or fusible resistor there.

Another point to consider would be range switching to allow better accuracy at low currents by only using one output stage. In the current circuit it gets possibly nonlinear below about 5 mV / 0.02 Ohms = 250 mA, as the OPs can have different offsets.

schmitt trigger:
Very interesting concept to include an external waveform generator input, to provide a a variable load to -let's say- test the a PSU stability.  :-+

The maximum frequency would, of course, be limited to the frequency response limited by the integrating capacitor in the feedback loop.

t1d:
Kleinstein, I was hoping you would drop by... You always have the good information and suggestions. Thanks, for your help. It is much appreciated.


--- Quote from: Kleinstein on September 19, 2018, 03:54:11 pm --- So those simple lever switches are OK.
--- End quote ---
Great, that's easy...


--- Quote from: Kleinstein on September 19, 2018, 03:54:11 pm ---One could consider a circuit that does not need the switch at all, if the internal and external signal are just added. So the internal reference would be still used to adjust the offset and leaving the input open is enough to disable an external signal.
--- End quote ---
Good trick...


--- Quote from: Kleinstein on September 19, 2018, 03:54:11 pm ---There should be no special need to use separate ground planes. It is more like one should avoid a ground plane at all, especially with the high current paths at the output.
--- End quote ---
I deleted the bottom ground plane, to lessen capacitance. It would be kind of difficult, to eliminate the top plane, because of the use of SMDs and dense populations. But, I will give it a look. I will use the bottom plane, for common feeds... power, load, etc. and separate them, with distance.


--- Quote from: Kleinstein on September 19, 2018, 03:54:11 pm ---This type of circuit is more suitable to use a star ground point. A ground plane is more for higher frequency and less suitable for precision.
--- End quote ---
I had to look up "star point" ground. I have been using it, all along, but didn't know it had a name.. lol


--- Quote from: Kleinstein on September 19, 2018, 03:54:11 pm ---A point too look for might be protection of the ground side of the F-gen input and the current monitoring output. These are connected to the power side and with wrong use could possibly carry high currents. So it might be a good idea to have something like a fuse or fusible resistor there.
--- End quote ---
Excellent idea... Maybe I will make their trace margins wider, too...


--- Quote from: Kleinstein on September 19, 2018, 03:54:11 pm ---Another point to consider would be range switching to allow better accuracy at low currents by only using one output stage. In the current circuit it gets possibly nonlinear below about 5 mV / 0.02 Ohms = 250 mA, as the OPs can have different offsets.
--- End quote ---
I have been thinking on this one, but I could use help, with the switching design... What type of switch, where to place them, etc...

From the op amp and MOSFET suggestions that you made, on the other thread, I decided on the TVL171 and IRFP250... Does that sound good (enough)?

I will be posting, the final design... So, please watch for it. But, there is still lots to do, first.

t1d:
Thanks, ST, for your help!


--- Quote from: schmitt trigger on September 19, 2018, 04:36:11 pm ---The maximum frequency would, of course, be limited to the frequency response limited by the integrating capacitor in the feedback loop.

--- End quote ---
I take it that you mean the cap in the feedback loop of the summing/driving op amp... Correct?

My FG only goes up to 20MHz. And, I really only intended to be using signals, to test power supplies. Do I just need to look on the internet, for a cap/frequency calculator? Or, is the concerns, for the needs of the summing/driving op amp circuit, too? Remember, I did not design the circuit... Jay_Diddy_B did...

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